Researchers probe how microbes speed up acid production at mining sites

Microbes are everywhere, but when they are in mined soils, they react with the mineral pyrite to speed up acidification of mine run-off water. Scientists have been trying to understand the chemistry behind this process that eventually leads to widespread acidification of water bodies and deposition of heavy metals. What a new study has found seems to defy the laws of chemistry: microbes react with the pyrite surface, coating it with chemicals that would be expected to hinder further reactions. Despite the formation of such coatings, however, microbe-mediated reactions occur tens of thousands of times faster than when no microbes are present.

’’That’s a puzzle,’’ said Alfred Spormann, a co-principal investigator on the study. ’’This changed surface chemistry should slow down the microbial oxidation but it doesn’t.’’

The collaborative study was led by co-principal investigators Scott Fendorf, Gordon Brown and Spormann at Stanford. Dartmouth Assistant Professor Benjamin Bostick, Fendorf’s former doctoral student who coordinated the research effort, will present the group’s findings Thursday, Dec. 11 at this year’s San Francisco meeting of the American Geophysical Union (AGU). The AGU is an international scientific society with more than 35,000 members dedicated to advancing the understanding of Earth and its environment.

In mines, oxygen from the air initiates chemical reactions with pyrite, also known as fool’s gold. Microbes subsequently react with the pyrite in cyclic processes that result in the rapid production of large amounts of sulfuric acid. The research team wanted to understand how the activity of the microorganisms controls the chemistry on mineral surfaces, and how that chemistry, in turn, controls the activity of the microorganisms. Specifically, they wanted to find out what kinds of iron species and precipitates can be found on microbe-treated pyrite surfaces.

The researchers grew the bacteria Thiobacillus ferrooxidans and Thiobacillus thiooxidans, forcing them to ’’eat’’ iron or sulfur, the elements that make up pyrite. They examined the products of metabolism using surface-sensitive photoelectron spectroscopy and X-ray absorption spectroscopy. The results give a molecular view of how microbes change the form of the mineral. The bacteria produced surface coatings made up of iron sulfate and the iron oxide goethite. Also, different metabolism products formed when both types of bacteria were studied together compared to when only one type was used. The amount of oxidation produced by the mixed species was not additive compared to oxidation by individual species, however. ’’The projects show that there is a fundamental difference between how one organism carries out a process and how a group does so,’’ said Bostick.

This study is a continuation of a pioneering molecular-level study by the same team, looking at how heavy metal contaminants partition between a biofilm and a metal surface. The researchers found then that surface type and metal concentration affect the distribution of the metal and the types of products formed.

The researchers will continue to experimentally reproduce and study the complex natural system of microbes and minerals, starting with simple systems and building complexity by sequentially adding more and different microorganisms.

The results may provide insight into other problems, such as tooth decay and metal-pipe corrosion, that arise from the interaction between microbes and the surfaces on which they reside.